Six percent of the solar energy reaching the earth's surface is ultraviolet (UV) radiation having a wavelength of 290-400 nanometers (nm). This radiation has two components:
(1) 5.5% UVA having a wavelength of 320-400 nm and PA1 (2) 0.5% UVB having a wavelength of 290-320 nm.
While the UV portion of the solar energy is relatively small, it induces nearly 99% of all the side effects of sunlight. UVB radiation, for example, is responsible for producing sunburn, aging and cancer of the skin. UVA radiation, for example, causes direct tanning and erythema (abnormal redness) and contributes to aging of the skin.
By avoiding exposure to sunlight, people can avoid the serious effects caused by the UV radiation. However, because of the nature of their work, some people cannot avoid exposure to the sun. In addition, others voluntarily expose their skin to the sun to tan, sometimes to extremes. Therefore, protection against the harmful effects of the sun is important.
Protection from these harmful effects of UV radiation exposure is available in the form of both topically applied formulations containing at least one physical blocker, or at least one chemical absorber, or combinations thereof. Physical blockers include active ingredients such as red petrolatum, titanium dioxide and zinc oxide. Chemical absorbers include active ingredients, such as para-aminobenzoic acid (more commonly known as PABA), which are generally transparent when applied and act by absorbing UV radiation, offering selective protection against certain UV wave bands, depending upon the absorption spectrum of the particular active ingredient incorporated into the formulation.
The effectiveness of a sunscreen formulation is generally assessed by how well it protects the skin in terms of a Sun Protection Factor (SPF) which is defined as the ratio of the amount of energy required to produce a minimal erythema on sunscreen protected skin to the amount of energy required to produce the same level of erythema on unprotected skin.
A number of the chemical absorbers and physical blockers, herein after referred to as "UV radiation absorbing agents," typically used in sunscreen formulations have adverse toxicological effects. Therefore, it is desirable to reduce the level of UV radiation absorbing agents present in a sunscreen formulation without reducing the level of protection.
One attempt to reduce the level of UV radiation absorbing agents in a sunscreen formulation is disclosed in U.S. Pat. No. 4,804,531 to Grollier, hereinafter referred to as "Grollier." Grollier discloses adding to a cosmetic screening composition an aqueous dispersion of water insoluble polymer particles where the polymer particles comprise a) an ionic polymer forming a core capable of being swollen, and b) a polymer forming a sheath at least partially encapsulating the core. The water insoluble polymer particles are disclosed to be film forming, to have a sheath glass transition temperature below 50.degree. C., and to have an average particle size before swelling of from 70 nanometers (nm) to 4500 nm. Grollier discloses that when the water insoluble polymer particles are added to a cosmetic screening composition at a level of from 0.1 to 10 weight percent, based on the total weight of the cosmetic screening composition, the absorption of UV radiation in the cosmetic screening composition is increased.
Improving upon the teachings of Grollier, we have unexpectedly found that voided latex particles having certain particle sizes, increase the absorbance of UV radiation in a composition containing one or more UV radiation absorbing agents.